The N-terminal extension of Arabidopsis ARGONAUTE 1 is essential for microRNA activities

microRNAs (miRNAs) regulate target gene expression through their ARGONAUTE (AGO) effector protein, mainly AGO1 in Arabidopsis thaliana. In addition to the highly conserved N, PAZ, MID and PIWI domains with known roles in RNA silencing, AGO1 contains a long, unstructured N-terminal extension (NTE) of little-known function. Here, we show that the NTE is indispensable for the functions of Arabidopsis AGO1, as a lack of the NTE leads to seedling lethality. Within the NTE, the region containing amino acids (a.a.) 91 to 189 is essential for rescuing an ago1 null mutant. Through global analyses of small RNAs, AGO1-associated small RNAs, and miRNA target gene expression, we show that the region containing a.a. 91–189 is required for the loading of miRNAs into AGO1. Moreover, we show that reduced nuclear partitioning of AGO1 did not affect its profiles of miRNA and ta-siRNA association. Furthermore, we show that the 1-to-90a.a. and 91-to-189a.a. regions of the NTE redundantly promote the activities of AGO1 in the biogenesis of trans-acting siRNAs. Together, we report novel roles of the NTE of Arabidopsis AGO1.

Introduction miRNA guide strand [22,23]; the Middle (MID) domain, which binds the 5' phosphate of the miRNA guide strand [24,25]; and the P-element-induced wimpy-testis (PIWI) domain, which in some AGOs, harbors an RNase H like motif that cleaves target RNA transcripts [26,27]. In Arabidopsis, AGO1, AGO2, AGO4, AGO7, and AGO10 have been shown to possess cleavage activity [7,15,[28][29][30]. Structural studies of human AGO2 and prokaryotic AGOs show that the four domains of AGO form a two-lobed structure with a central cleft that cradles guide and target RNAs, with the N-PAZ domains composing one lobe and the MID-PIWI domains constituting the other [26,31]. The N and PAZ domains are connected by the L1 linker, while the PAZ and MID domains are connected by the L2 linker, which also connects the N-PAZ lobe and the MID-PIWI lobe [31][32][33][34].
In addition to the N, PAZ, MID and PIWI domains that are highly conserved, AGO proteins may harbor an N-terminal extension (NTE) of varying lengths and sequences, and the molecular or biological functions of this region are less understood. The NTEs of AGOs are also referred as the N-terminal coil as they were predicted to possess coil-like structures [35]. A nuclear localization signal (NLS) and a nuclear export signal (NES) are present in the NTE (1-189a.a.) of Arabidopsis AGO1 [36]. As inhibition of the EXPO1/NES-dependent protein nuclear export pathway significantly increases the ratio between nuclear and cytoplasmic AGO1, it was proposed that these signals direct the nucleo-cytosolic shuttling of AGO1 [36]. Strikingly, AGO1 with the NES sequence mutated (AGO1mNES) associated with the same miRNA cohorts as its intact counterpart. This led to the proposal that the loading of miRNAs into AGO1 takes place in the nucleus and the NES mediates the nuclear export of miRISCs [36]. However, there are other nuclear export pathways that transport AGO1 from the nucleus to the cytoplasm. The TREX-2 complex core component THP1 partners with the nucleoporin protein NUP1 at the nuclear envelope, together promoting the nuclear export of AGO1 or AGO1-miRISC [37]. Furthermore, the possibility of cytoplasmic AGO1 loading cannot be excluded. In fact, a cytoplasmically-sequestered AGO1 protein was shown to load miR165/6 in vivo [38]. 56 mutant alleles of Arabidopsis AGO1 have been reported in the literature, with 20 of the 56 being missense mutations, however only one of the 20 resides in the NTE [35]. ago1-38, a weak allele of AGO1, harbors a G186R mutation at the very end of the NTE region [39]. One study reported that the AGO1 protein abundance is similar between ago1-38 and wild-type (WT), but the membrane association of AGO1 in the inflorescence tissue is reduced in ago1-38 [40]. Furthermore, AGO1 is under feedback regulation-AGO1 RNA is targeted by AGO1-bound miR168 with the miR168-binding site being located in the region encoding the NTE [41]. Recent studies showed that AGO1 residues K178, K185 and K190 are required for interaction with autophagy cargo receptors ATI1 and ATI2, and mutations of these residues reduce the degradation rate of unloaded AGO1 in vivo [42,43]. Studies in animal AGOs show diverse functions of the NTE region. In Caenorhabditis elegans, WAGO-1 (a worm ARGONAUTE) and WAGO-3 are processed at their NTE by DPF-3, a dipeptidase [44]. Proteolytic activity of DPF-3 on the third and second a.a. of WAGO-1 and WAGO-3, respectively, promotes the correct sorting of 22G siRNAs into these AGOs and thus safeguard genome integrity [44]. Drosophila melanogaster AGO2 contains a~400 a.a. long N-terminal glutamine-rich repeat (GRR) region, and reducing GRR copy numbers results in defects in RNAi responses and embryonic development [45].
Here, through analyzing the miRNA-related defects manifested by the Arabidopsis AGO1 NTE truncation lines, we uncover novel roles of the NTE in miRNA-mediated gene silencing. We show that loss of the entire NTE leads to a seedling-lethal phenotype resembling that of an ago1 null allele, suggesting that the NTE is essential for the functions of AGO1. Additionally, we find that the 91-to-189a.a. region of the AGO1 NTE is essential for rescuing the morphological defects of the ago1 null allele. Global analyses of miRNAs and their association with AGO1 revealed that the NTE, and particularly the 91-189 region, facilitates miRNAs' loading into AGO1. Truncation of AGO1 1-to-90a.a. reduces its nuclear partitioning without affecting its profiles of miRNA and ta-siRNA association. Furthermore, we show that the biogenesis of ta-siRNAs requires the presence of either the 1-to-90a.a. or the 91-to-189a.a. region of AGO1. Taken together, our results reveal novel roles of the NTE of Arabidopsis AGO1.

Truncation of the AGO1 N-terminal extension causes severe morphological defects
In Arabidopsis thaliana, the ten AGO family members can be grouped into three clades based on their phylogenetic relationship (S1A Fig) [46]. All members of the AGO1/5/10 and AGO2/ 3/7 clades possess NTEs longer than 130 a.a., whereas members of the AGO4/6/8/9 clade contain NTEs less than 70 a.a. long (S1A Fig). Protein sequence alignment of the NTE region of all Arabidopsis AGOs shows extremely low similarity (S1B Fig). Even AGO10, the most closely related paralog of AGO1, shares little similarity in its NTE region to that of AGO1. Furthermore, phylogenetic analysis using only the NTE regions of the ten AGOs shows that the AGO10 NTE is the most distantly related to that of AGO1 (S1C Fig). Together, the sequence analyses suggest that the NTE region of AGOs might serve AGO-specific functions.
To investigate the functions of the NTE (1-189a.a.) region of Arabidopsis AGO1, we searched for potential domains and functional motifs in this region by employing protein databases including Pfam, InterPro, and NCBI Conserved Domains Database. The 75-to-172 a.a region of Arabidopsis AGO1 was annotated as a glycine-rich_AGO1 domain by multiple protein databases. This domain appears in 117 species, all within the Magnoliopsida class (flowering plants), suggesting a unique function to flowering plants. Sequence alignment of the AGO1 NTE from six Magnoliopsida species, including Arabidopsis thaliana (At), Arabidopsis lyrate (Al), Brassica napus (Bn), Glycine max (Gm), Oryza sativa (Os), and Zea mays (Zm), shows that this glycine-rich_AGO1 domain is evolutionally conserved (S2 Fig). In addition, six RGG/RG repeats are found within the NTE region. The RGG/RG motif, which may undergo arginine methylation, is known for mediating RNA binding, protein localization, and protein-protein interactions [47]. Three of the six RGG/RG motifs are clustered at a.a. 83 to 103 in the glycine-rich_AGO1 domain, with two of them being conserved in AGO1s from the six species mentioned above except for Oryza sativa. One RGG/RG motif, 59 RGG 61 , is conserved in all six species. Furthermore, glutamine (Q) content is particularly high in the NTE, with 30 Qs scattered along the region and accounting for 15.9% of the total amino acids in the NTE. This feature is conserved in the NTEs of all six inspected plant AGO1s. Next, we predicted sorting signals within the NTE that might facilitate the sub-cellular localization of AGO1. In addition to an NLS ( 2 VRKRR 6 ) and NES ( 149 LAQQFEQLSV 158 ) that were previously reported for Arabidopsis AGO1 [36], a putative NLS ( 102 GGGPSSGPPQ 111 ) was predicted by SeqNLS with a score of 0.737 out of 1.
To understand how these features of the NTE affect the functions of AGO1, we tried to predict the structure of the NTE region by searching for similar protein sequences with available structural information in the Protein Data Bank using the SWISS-MODEL server. However, no protein template with significant similarity to AGO1 NTE was found. Next, we utilized AlphaFold, a machine learning tool that predicts protein structures, to get insights into the 3D structure of the NTE region ( Fig 1A). The N-terminal segment (a.a 1-90) of the NTE is mostly unstructured and loosely attached to the rest of the AGO1 protein. The C-terminal segment (a. a. 91-189) of the NTE is more structured and could potentially interact with the PIWI domain of AGO1. The very C-terminus of the AGO1 NTE together with the L2 Linker is tucked in between the N and PIWI domains, and thus could potentially assist with the connection and movement between the N-PAZ lobe and the MID-PIWI lobe. The rest of the NTE C-terminus protrudes from the two lobes of AGO1, with a.a. 145-175 adopting an L shape that is close to the PIWI domain.
To further investigate how the NTE region affects the function of Arabidopsis AGO1 in vivo, we introduced N-terminal HA-tagged, wild-type (WT) or truncation mutants of AGO1 without the NTE region (AGO1Δ1-189), without the unstructured N-terminal segment of the NTE (AGO1Δ1-90), or without the more structured C-terminal segment of the NTE (AGO1Δ91-189) into the ago1-36 mutant background (Fig 1B). The ago1-36 allele contains a T-DNA insertion in the AGO1 gene at a position within the encoded PAZ domain [7]. The phenotype of ago1-36 resembles ago1 null alleles, such as ago1-3 [48,49]. HA-AGO1Δ1-90 ago1-36 line #29 and HA-AGO1 ago1-36 line #20 were selected for downstream analysis because they had similar levels of AGO1 protein (S3

PLOS GENETICS
The N-terminal extension of Arabidopsis ARGONAUTE 1 from HA-AGO1Δ91-189 and HA-AGO1Δ1-189 were likely due to the lack of regulation of AGO1 mRNA by miR168, as the miR168 target site was removed from these transgenes. We found that both HA-AGO1 ago1-36 and HA-AGO1Δ1-90 ago1-36 had a WT phenotype ( Fig  1C), suggesting that the N-terminal half of the NTE is largely dispensable, at least under laboratory growth conditions. HA-AGO1Δ91-189 partially rescued the developmental phenotypes of ago1-36 plants, whereas HA-AGO1Δ1-189 ago1-36 seedlings resembled ago1-36 except that the hypocotyl hook was straightened, and the cotyledons were expanded at early developmental stages (Fig 1C). This suggests that the NTE, and particularly the region of a.a. 91-189, is essential for the functions of AGO1.

The a.a. 91-189 region of the AGO1 NTE is crucial for miRNA accumulation
To determine the effects of AGO1 NTE truncation on miRNA accumulation, we performed RNA gel blot assays to determine the levels of miRNAs in seedling of ago1-3, ago1-36, and ago1-36 expressing HA-tagged wild-type or truncated forms of AGO1. We found that compared to AGO1 ago1-36, the levels of three tested miRNAs were decreased in AGO1Δ91-189 ago1-36 and AGO1Δ1-189 ago1-36 such that they were similar to those of ago1-36 and ago1-3, whereas their levels remained unchanged in AGO1Δ1-90 ago1-36 (Fig 2A). Next, we used small RNA sequencing to examine the effects of AGO1 NTE truncations on miRNA accumulation. Principal component analysis (PCA) of the small RNA sequencing data showed that the three biological replicates of each genotype were highly reproducible (S4A Fig). The size distribution of both total small RNAs and miRNAs was similar in ago1-36 and ago1-36 expressing wild-type or NTE-truncated AGO1s (S4B Fig). Consistent with previous findings, total small RNAs showed a 21-nucleotide (nt) peak and a more abundant 24-nt peak, while the majority of miRNAs were 21-nt long (S4B Fig) [10,29].
Detailed analyses were carried out to compare the abundance of individual miRNAs between AGO1 NTE truncation lines and the line expressing full-length AGO1. Among a total of 428 Arabidopsis miRNAs, 104 miRNAs and 53 miRNA � s with an average level > 2 RPM (reads per million) in the small RNA-seq samples were included in the analyses. Few differentially accumulated miRNAs and miRNA � s were found between AGO1Δ1-90 ago1-36 and AGO1 ago1- 36, with examples being miR165a/b, miR168a/b, miR171b/c and miR390a/b (Fig 2B and S1 Table). On the other hand, more than 36% of miRNAs were decreased and over 28% of miRNA � s were increased in AGO1Δ91-189, AGO1Δ1-189, and ago1-36 when compared to AGO1 ago1-36 ( Fig 2B and S1 Table). This result was consistent with miRNA quantification using RNA gel blot assays (Fig 2A). In accordance with the previous report on ago1 -3 [49], a null allele of AGO1, around 54% (56 out of 104) of miRNAs were significantly decreased in ago1-36 compared to AGO1 ago1-36; it is likely that failure of these miRNAs to load into an AGO protein made them more vulnerable to degradation. In contrast, 7 miRNAs were upregulated in ago1-36 compared to AGO1 ago1-36, for example, miR172a;b, miR866, and miR5026. These upregulated miRNAs might be preferentially sorted into and stabilized by AGO2, as 5 out of 7 upregulated miRNAs possess a 5' adenine (A), the preferred 5' nucleotide by Arabidopsis AGO2 [8]. In addition, 15 miRNA � s accumulated to a higher level in ago1-36 compared to AGO1 ago1-36, for example, miR391 � and miR393a/b � , both of which containing a 5' adenine (A) and shown to preferentially associate with AGO2 [8]. As the majority of the upregulated miRNA � s in ago1-36 contain a 5' adenine (A) or guanine (G), they might be selectively loaded and protected by other AGOs when AGO1 is absent.
Comparison showed that the differentially accumulated miRNAs and miRNA � s (i.e. reduced levels of miRNAs and increased levels of miRNA � s) in ago1-36 were highly correlated s in AGO1 ago1-36 (X axis) and fold change between AGO1 mutants and AGO1 (Y axis). Red and blue dots denote miRNAs or miRNA � s with significantly increased and decreased abundance (average RPM > 2, fold change > 1.5 and adjust P value < 0.01), respectively. (C) Scatter plots comparing the log 2 (fold change) of miRNAs and miRNA � s between pairs of genotypes. Pearson's correlation with those in AGO1Δ1-189 ago1-36, and with those in AGO1Δ91-189 ago1-36, although to a lesser extent (Fig 2C). No correlation was found for the differentially accumulated miRNAs and miRNA � s in ago1-36 and AGO1Δ1-90 ago1-36 ( Fig 2C). These findings suggest that the AGO1 NTE (1-189a.a.), particularly the 91-189 region, is required for restoring miRNA and miRNA � levels in ago1-36. And as the differential accumulation of a small number of miRNAs and miRNA � s between AGO1Δ1-189 ago1-36 and AGO1Δ91-189 ago1-36 was not concordant, the a.a 1-90 region of the NTE might also play a minor role in miRNA accumulation.
To exam whether miRNA-miRNA � pairs are affected simultaneously in AGO1 NTE mutants, a total of 31 miRNA species with both miRNA and miRNA � passing the abundance filter (average RPM > 2) in the small RNA sequencing data were selected for evaluation. The levels of most miRNAs and their miRNA � s in ago1-36 showed an inverse trend-while the majority of the miRNAs were downregulated, their corresponding miRNA � s were up-regulated, likely due to association with other AGOs (Fig 2D). Only a few miRNAs showed the same trend in changes in ago1-36 to that of their corresponding miRNA � s, such as miR172a;b; c;d and miR390a;b ( Fig 2D). A similar pattern was observed in AGO1Δ1-189 ago1-36 and AGO1Δ91-189 ago1-36, indicating miRISC formation was likely defective for both forms of AGO1, and the a.a 91-189 region of AGO1 might be important in miRISC formation ( Fig 2D).

The a.a. 91-189 region of the AGO1 NTE is essential for miRNA and ta-siRNA loading
To test our hypothesis that the NTE region of AGO1 affects miRISC formation, we examined whether the association between miRNAs and ta-siRNAs with the NTE-truncated AGO1 is compromised. Small RNAs associated with the HA-tagged AGO1 and AGO1 mutants were immunoprecipitated (IP-ed) and sequenced. For these experiments, the AGO1Δ91-189 and AGO1Δ1-189 transgenes were in the ago1-36 heterozygous background to ensure a similar cellular small RNA profile and morphological phenotypes across all tested plants. PCA analysis showed that the two biological replicates for each genotype were highly reproducible, and AGO1Δ91-189 IP and AGO1Δ1-189 IP samples were clustered together and separate from the other samples, suggesting they had small RNA profiles similar to each other yet different from AGO1 IP and AGO1Δ1-90 IP (S5A Fig Consistent with previous findings [8], the size distribution of wild-type AGO1-associated total small RNAs, miRNAs, and ta-siRNAs showed a 21-nt peak ( Fig 3A). Like AGO1, AGO1Δ1-90 predominantly associated with 21-nt small RNAs, however, a minor reduced preference toward 21-nt miRNAs and an increased association with 20-nt miRNAs were observed ( Fig 3A). As the sequenced size of small RNAs was used in the size distribution analysis, the 20-nt miRNAs could represent a pool of annotated 20-nt miRNAs, truncated miRNAs from a larger size, and misprocessed miRNAs. We specifically analyzed the association of miR-NAs with various annotated sizes with AGO1. Annotated 20-nt miRNAs constituted a slightly higher proportion in AGO1Δ1-90-associated miRNAs as compared to wild-type AGO1, suggesting that the a.a. 1-90 region of the NTE might facilitate size selection of miRNAs (S5B Fig). Strikingly, AGO1Δ91-189 and AGO1Δ1-189 completely lacked the preference toward coefficients (R) and p-values are indicated for miRNAs and miRNA � s separately. (D) Heatmap depicting the levels of miRNAs and their corresponding miRNA � s in AGO1 mutants relative to pAGO1:3XHA-AGO1 ago1-36. Note that only miRNAs for which the corresponding miRNA � s also passed the abundance filter (average RPM > 2) are included in the heatmap. https://doi.org/10.1371/journal.pgen.1010450.g002 21-nt small RNAs, and their association with miRNAs and ta-siRNAs was minimal, suggesting that the a.a. 91-189 region of AGO1 is essential for miRNA and ta-siRNA loading (Fig 3A).
We next examined the differential association with individual miRNAs between AGO1 NTE mutants and wild-type AGO1. A total of 76 miRNAs and 24 miRNA � s were at an average level of > 2 RPM in the IP small RNA-seq samples and were included in the analysis (S2 Table). The association of miRNAs and miRNA � s with AGO1Δ1-90 was unaffected compared to wild-type AGO1, suggesting that a.a 1-90 of AGO1 have negligible effects on miRNAs s with significantly different levels (fold change > 1.5 and adjust P value < 0.01) between IP-ed AGO1 mutants and AGO1. Red dots denote miRNAs or miRNA � s with significantly higher abundance, while blue dots denote miRNAs or miRNA � s with significantly lower abundance. (C) Bar plots depicting the log 2 (fold change) of 21-nt ta-siRNAs between IP-ed AGO1 NTE-truncated forms and AGO1. Blue bars denote ta-siRNAs with significantly lower levels (fold change > 1.5 and adjust P value < 0.01). (D) Quantification of the nuclear-cytoplasmic partitioning of AGO1. Western blots were performed with various genotypes as indicated. Blots were probed using AGO1, HA, or GFP antibodies to detect AGO1. Histone H3 is a nuclear marker and used to quantify AGO1 in the total lysate (T) and the nuclear fraction (N). cFBPase is a cytoplasmic marker and used to quantify AGO1 in the T and the cytoplasmic fraction (C). The numbers above the bands represent relative protein levels. Bar plots summarize results of cytoplasmic/nuclear ratios of AGO1 from several experimental repeats shown here and in S7 Fig. Two replicates of wild-type AGO1 and two replicates of AGO1Δ1-90 were in the ago1-36 background, AGO1mNES was in the Col background, while other AGO1 NTE-truncated mutants, two replicates of AGO1Δ1-90, and two replicates of wild-type AGO1 were in the Col-0 background. Error bars indicate s.d.
https://doi.org/10.1371/journal.pgen.1010450.g003 loading ( Fig 3B). On the contrary, more than 88% of miRNAs and 87% of miRNA � s showed reduced association with both AGO1Δ91-189 and AGO1Δ1-189 (Fig 3B), and the miRNAs and miRNA � s downregulated in the two mutants were highly correlated (S5C Fig), suggesting that the 91-189 a.a. region of AGO1 affects miRNA association globally. Furthermore, both the miRNA strand and its corresponding miRNA � strand showed reduced association with AGO1Δ91-189 and AGO1Δ1-189 (S5D Fig), implying that the reduced miRNA loading is unlikely due to defects in stand selection or miRNA � ejection. As an AGO protein stabilizes its associated miRNAs, this compromised miRNA loading could explain the severe phenotypes and the reduced abundance of miRNAs in AGO1Δ91-189 ago1-36 and AGO1Δ1-189 ago1-36. The reduced association of miRNA � s with AGO1Δ91-189 and AGO1Δ1-189 also suggests that the increased accumulation of miRNA � s in AGO1Δ91-189 ago1-36 and AGO1Δ1-189 ago1-36 is likely due to the loading of miRNA � s into other AGOs. Similar to miRNAs, most 21-nt ta-siRNAs showed reduced levels in AGO1Δ91-189 IP and AGO1Δ1-189 IP, but not in AGO1Δ1-90 IP (Fig 3C).
Although miRNAs and ta-siRNAs were almost depleted in AGO1Δ91-189 IP and AGO1Δ1-189 IP, these proteins were still associated with small RNAs in vivo (Fig 3A). Wildtype AGO1 was mainly associated with 21-nt miRNAs and ta-siRNAs (Figs 3A and S6A), although it was also loaded with Pol IV-dependent siRNAs and small RNAs derived from coding genes, TEs, repeats, snRNA, snoRNA, rRNA, and tRNA (S6A Fig). We found that the proportion of rRNA-and tRNA-derived small RNAs was dramatically increased in AGO1Δ91-189 IP and AGO1Δ1-189 IP compared to wild-type AGO1 IP, while all other small RNA species showed a corresponding decrease ( The 91-to-189 a.a region of AGO1 facilitates miRNA loading independently of its role in nuclear-cytoplasmic shuttling AGO1 was previously reported to shuttle between the cytoplasm and the nucleus in an NLS (a. a. 2-6)-and NES (a.a. 149-158)-dependent manner [36]. The AGO1Δ1-90 protein lacks the NLS (a.a. 2-6), and the NES (a.a. 149-158) was removed from AGO1Δ91-189, while the AGO1Δ1-189 lacks both the NLS and the NES. To investigate whether the reduced loading of miRNAs and ta-siRNAs into AGO1 NTE mutants was due to defects in nuclear-cytoplasmic shuttling, we further examined the nucleocytoplasmic distribution of AGO1 in these mutants. In addition to the NTE truncation mutants generated in this study, we also included an NES mutated AGO1 (AGO1mNES) allele that was reported before, which showed enhanced nuclear localization, intact miRNA association, and reduced ta-siRNA binding [36]. Consistent with previous findings [36,37], the cytoplasm/nucleus ratio of wild-type AGO1 was around 3:1, and this ratio was greatly reduced for the AGO1mNES protein (Figs 3D and S7). In comparison to wild-type AGO1, the levels of cytoplasmic AGO1Δ1-90 increased, supporting the nucleus-importing role of NLS (a.a. 2-6) that resides within the 1-to-90 a.a. region of the NTE. However, the reduced levels of nuclear AGO1Δ1-90 did not affect its profiles of miRNA and ta-siRNA association (Fig 3B). Interestingly, the cytoplasm/nucleus ratio of the AGO1Δ1-189 protein lacking both NLS (a.a. 2-6) and NES (a.a. 149-158) was similar to that of wild-type AGO1, suggesting that there exist mechanisms to affect the nuclear-cytoplasm shuttling of AGO1 in an NTE-independent manner. Surprisingly, AGO1Δ91-189, despite lacking the NES (a.a. 149-158) that directs AGO1's nuclear export, was not as affected as AGO1mNES in cytoplasmic localization (Fig 3D). One possible explanation is that an unidentified motif in a.a. 91-189 of AGO1 facilitates the nuclear import. In this study, we identified a putative NLS ( 102 GGGPSSGPPQ 111 ) that resides in this region of AGO1, but its role in nuclear import has yet to be tested. It is worth noting that, unlike AGO1mNES, which predominantly resides in the nucleus and shows a wild-type-like miRNA-binding profile, AGO1Δ91-189 even though its nuclear accumulation is increased compared to wild-type AGO1, its miRNA association is almost depleted, suggesting that a.a. 91-189 of AGO1 regulate miRNA loading independently of its role in the nuclear-cytoplasmic partitioning of AGO1 (Fig 3D).

The 1-90 a.a. and 91-198 a.a. regions of the AGO1 NTE function redundantly in ta-siRNA biogenesis
To test whether the NTE region of AGO1 affects the activity of miRISC, we performed RNAseq to examine the expression of miRNA target genes (S3 Table). The same RNA samples that were used in the small RNA sequencing analysis were also used for the RNA-seq analysis, including ago1-36 and ago1-36 expressing wild-type AGO1 or AGO1 NTE truncated mutants driven by the AGO1 promoter. Three biological replicates gave highly reproducible results (S8 Fig). 130 experimentally verified miRNA target genes were investigated to identify differentially expressed genes between wild-type AGO1 and AGO1 NTE mutants. In comparison to AGO1 ago1-36, gene expression in AGO1Δ1-90 ago1-36 was largely unaffected (Fig 4A and  4B). Around 35% of miRNA target genes were derepressed in ago1-36 and AGO1Δ1-189 ago1-36 (Fig 4A), and the two genotypes were highly correlated in terms of the miRNA target genes affected ( Fig 4C). Interestingly, only 25% of miRNA target genes were derepressed in AGO1Δ91-189 ago1-36 (Fig 4A), and the levels of change were lesser than their counterparts in ago1-36 and AGO1Δ1-189 ago1-36 ( Fig 4D). The less severe molecular phenotype of AGO1Δ91-189 ago1-36 is consistent with their better seedling phenotype compared to AGO1Δ1-189 ago1-36 (Fig 1C), suggesting that a.a. 1-90 of AGO1 can partially rescue AGO1Δ1-189 ago1-36. Since miRNA association is reduced to a similar level in AGO1Δ91-189 and AGO1Δ1-189, it is possible that a.a. 1-90 of AGO1 facilitate target repression after miRNA loading.
In addition to target gene repression, certain miRNAs can trigger the biogenesis of secondary siRNAs from target loci such as TAS1a/b/c and TAS2, which produce ta-siRNAs in an AGO1-miR173 dependent manner. To determine whether the NTE of AGO1 affects ta-siRNA biogenesis, we compared the level of 21-nt small RNAs derived from the TAS loci and their phasing pattern. TAS3a/b ta-siRNAs, which require AGO7-miR390 for biogenesis, were included as a control. Strikingly, by small RNA-seq we found that ta-siRNA levels from the four AGO1-dependent loci were largely unchanged in AGO1Δ1-90 ago1-36 or AGO1Δ91-189 ago1-36 while the ta-siRNA levels were significantly reduced in ago1-36 and AGO1Δ1-189 ago1-36 (Fig 4E). In fact, the small RNA phasing patterns were similar in ta-siRNAs were unaffected in AGO1Δ1-189 ago1-36 (Fig 4E), consistent with expectations. We further confirmed that the reduced ta-siRNA levels from TAS1 and TAS2 loci in AGO1Δ1-189 ago1-36 were not due to reduced accumulation of miR173, as the levels of miR173 were largely unaffected (Fig 4F). Both AGO1Δ91-189 and AGO1Δ1-189 showed reduced association with miR173 (Fig 4G), which raised the possibility that miR173 triggered the biogenesis of ta-siRNAs in association with another AGO. However, this cannot explain the lack of ta-siRNA biogenesis in AGO1Δ1-189 ago1-36. A more likely scenario is that AGO1Δ91-189, despite reduced miR173 association, is sufficient to trigger ta-siRNAs production, while AGO1Δ1-189 cannot, which would suggest that a.a. 1-90 play a role in ta-siRNA biogenesis. However, ta-siRNA accumulation was not affected in AGO1Δ1-90 ago1-36, implying that a.a. 91-189 can also facilitate ta-siRNA biogenesis. It is likely that a.a. 1-90 and a.a. 91-189 of AGO1 redundantly enable the activities of AGO1 in ta-siRNA biogenesis.

Discussion
In this study, we show that the NTE (1-to-189 a.a.) region of AGO1 is essential for rescuing the developmental and molecular phenotypes of ago1-36. We further show that the a.a. 91-189 region is required for AGO1's association with miRNAs and ta-siRNAs in vivo independently of its role in AGO1's nuclear-cytoplasmic shuttling. On the other hand, the a.a 1-90 region, which contains the NLS (a.a. 2-6), is dispensable for AGO1's association with miRNAs and ta-siRNAs in vivo, but plays a role in target RNA regulation. While the NTE is essential for ta-siRNA biogenesis, the presence of either a.a. 1-90 or a.a. 91-189 of the NTE is sufficient for ta-siRNA biogenesis, suggesting a redundant function of these two regions in ta-siRNA biogenesis. Below, we discuss how results from this study support these conclusions.
Proper loading into an AGO protein is essential for miRNAs to exert their roles in target gene repression. We show that miRNA association is severely and similarly compromised for AGO1Δ91-189 and AGO1Δ1-189 (Figs 3A, 3B, and S5C), suggesting a.a. 91-189 of AGO1 are required for miRNA loading. AGO1Δ1-90 IP shows a similar miRNA profile to wild-type AGO1 IP, suggesting that a.a. 1-90 of AGO1 are not required for miRNA loading (Fig 3A and  3B). A previous study shows that an NLS (a.a. 2-6) in AGO1 promotes the nuclear localization of a reporter protein (GFP-GUS) when a portion of the AGO1 NTE (a.a. 1-148) is fused to the reporter protein, while mutating an NES (a.a. 149-158) prevents the nuclear export of AGO1 [36]. It was proposed that miRNA loading into AGO1 occurs inside the nucleus, as AGO1mNES, which predominantly resides in the nucleus, shows a similar miRNA binding profile to that of wild-type AGO1 [36]. In this study we show that AGO1Δ91-189, which is unaffected or even increased in nuclear localization (Fig 3D), does not manifest a wild-typelike miRNA-binding profile (Fig 3B) as does AGO1mNES [36], suggesting that a.a. 91-189 of AGO1 are critical to miRNA loading independently of effects on the nuclear-cytoplasmic shuttling of the protein. It is possible that a.a. 91-189 of AGO1 are required for the interaction with HSP90, which is essential for miRNA loading [50]. Furthermore, we show that the reduced nuclear localization of AGO1Δ1-90 does not affect its miRNA loading or target transcripts repression (Figs 3B, 3D, and 4A), suggesting that the residual nuclear localization was sufficient to allow for miRNA loading or that miRNA loading can also occur in the cytoplasm. Indeed, miR165/6 can be loaded onto cytoplasmically-sequestered AGO1 in vivo [38]. Unlike AGO1mNES, which almost exclusively resides in the nucleus, approximately 30% of the AGO1Δ91-189 protein is in the cytoplasm (Fig 3D), suggesting a motif that promotes nuclear import could be in the deleted region. An NLS ( 102 GGGPSSGPPQ 111 ) that resides in the a.a. 91-189 region of AGO1 was predicted in this study. Mutating both NLS (a.a. 2-6) and the putative NLS (a.a. 102-111) might create a cytoplasm-exclusive AGO1 that is suitable to test our hypothesis.
The 1-to-90 a.a. region of AGO1 is not required for rescuing the developmental defects of ago1-36 or for miRNA loading, however, we found it can facilitate target repression and ta-siRNA biogenesis. We show that the association of miRNAs with AGO1Δ91-189 and AGO1Δ1-189 is almost abolished (Fig 3A and 3B), whereas miRNA target gene repression is less affected in AGO1Δ91-189 ago1-36 compared to AGO1Δ1-189 ago1-36 (Fig 4A and 4D), implying that a.a. 1-90 of AGO1 assist in target transcript cleavage. For ta-siRNA biogenesis from TAS1 and TAS2 loci, both AGO1Δ1-90 ago1-36 and AGO1Δ91-189 ago1-36 resemble AGO1 ago1-36 in terms of ta-siRNA levels and phasing patterns, while AGO1Δ1-189 ago1-36 and ago1-36 are defective in ta-siRNA biogenesis (Figs 4E and S9A). Although miR173, the trigger miRNA for the biogenesis of TAS1a/b/c-and TAS2-derived ta-siRNAs, was significantly reduced in AGO1Δ91-189 IP compared to wild-type AGO1 IP (Fig 4G), it is unlikely that other AGOs associate with miR173 to trigger ta-siRNA biogenesis when AGO1 loading is compromised, as miR173-dependent ta-siRNA biogenesis is greatly affected in ago1-36 ( Fig  4E). We hypothesize that only a small amount of AGO1-miR173 is required to trigger the biogenesis of ta-siRNAs in AGO1Δ91-189 ago1-36. Together, these observations suggest that a.a. 1-90 of the NTE are sufficient for ta-siRNA biogenesis.
Another interesting observation was that AGO1Δ91-189 ago1-36 was defective in pha-siRNA biogenesis, as evidenced by reduced siRNA abundance at three PPR genes and lack of siRNA phasing at two NBS-LRR genes (Figs 4H and S9C). The trigger miRNAs for ta-siRNA and phasiRNA biogenesis, such as miR173, miR161.1, and miR472, all show reduced association with AGO1Δ91-189 ( Fig 4G and S2 Table), but why is AGO1Δ91-189 capable of ta-siRNA biogenesis but not phasiRNA biogenesis from protein-coding genes? One possibility is that these two processes require different auxiliary proteins, and a.a. 91-189 are required for specific protein-protein interactions. Alternatively, these two processes might take place at distinct subcellular locations, and a.a. 91-189 are required for the localization of AGO1 to one of the subcellular compartments.

Plant materials and growth conditions
All Arabidopsis thaliana lines in this study were in the Columbia (Col) ecotype. Seeds of ago1-36 (SALK_087076) were obtained from the ABRC collection. Seeds were surface-sterilized with 75% ethanol and stratified in water at 4˚C for 2 days then transferred to 1x Murashige-Skoog (MS) medium. All plants were kept in a growth chamber at 22˚C under full-spectrum white light and long-day conditions (16 h light / 8 h dark).
ago1-36 and ago1-36/+ expressing pAGO1::3XHA-AGO1, pAGO1::3XHA-AGO1Δ1-90, pAGO1::3XHA-AGO1Δ91-189, and pAGO1::3XHA-AGO1Δ1-189 were generated in this study. To construct the plasmids of the above transgenes, a 1648bp AGO1 promoter (including the 5' UTR) and the 399bp 3' UTR were amplified from genomic DNA, the full length AGO1 coding region and NTE-truncated AGO1 were amplified from cDNA, and the 3xHA tag was amplified from pGWB615 [51]; these fragments were then cloned into pEarleyGate 301 [52] (predigested by XbaI and NcoI) using the NEBuilder HiFi DNA Assembly Cloning Kit (NEB). All clones were validated by sequencing. Sequences of the primers are listed in S4 Table. Small RNA gel blot analysis and sequencing RNA isolation and RNA gel blot analysis of small RNAs were performed as described [10]. 5-10 μg of total RNA was isolated from 12-day-old Arabidopsis seedlings using TRI Reagent (NRC), resolved in 15% urea-PAGE gels and transferred to NX membranes (Amersham Hybond-NX). MiRNAs were detected with 5' end 32 P-labeled antisense DNA oligonucleotide probes. Sequences of the DNA oligonucleotide probes in this study are listed in S4 Table. Hybridization signals were analyzed by phosphorimager (Typhoon 9410, GE).
Small RNA sequencing and data analysis were performed as described [10]. 25 μg of total RNA was extracted from 12-day-old Arabidopsis seedlings and resolved in a 15% urea-PAGE gel, and small RNAs of 15-to 40-nt were isolated from the gel. Small RNA libraries of gel-purified small RNAs or small RNAs acquired from AGO1 immunoprecipitation (as described below) were constructed using the NEBNext Multiplex Small-RNA Library Prep Set for Illumina (E7300). The libraries were sequenced on an Illumina HiSeq X Ten, and the resulting data were analyzed using an in-house pipeline pRNASeqTools v.0.8 (https://github.com/ grubbybio/pRNASeqTools). The three-prime-end adapter sequence (AGATCGGAAGAGC) was trimmed from raw reads, followed by filtering to retain 18-to 42-nt reads using cutadapt v3.0 [53]. Trimmed reads were then aligned to the Arabidopsis genome (TAIR10) using Short-Stack v. 3.8.5 [54] with parameters '-bowtie_m1000 -ranmax 50 -mmap u -mismatches 0'. For total small RNAs, reads were normalized against total 18-to 42-nt reads minus 45S rRNA reads (RPM, Reads Per Million Reads). For small RNAs acquired from AGO1 immunoprecipitation, reads were normalized against IP-ed 18-to 42-nt reads (RPM). Differential comparison of small RNAs was conducted by DESeq2 v1.30.0 with fold change of 1.5 and adjusted P value < 0.01 as the parameters [55]. Annotation of miRNA and miRNA � sequences was obtained from miRbase v21 (http://www.mirbase.org/). For miRNA length distribution and small RNA composition analysis, reads that match perfectly or with a 1-nt shift on either end from the annotated sequence were assigned to the miRNA. For miRNA differential expression analysis, only reads that match perfectly to the annotated sequence were assigned to the corresponding miRNA. For ta-siRNA, levels were quantified by summing 21-nt small RNA reads that mapped to each of the eight TAS loci.

Immunoprecipitation and protein gel blot analysis
2 g of 12-day-old seedlings was ground into fine powder in liquid nitrogen and the powder was homogenized in 3 ml lysis buffer (50 mM Tris-HCl, pH7.5, 150 mM NaCl, 10% glycerol, 0.1% CA-630, one tablet of cOmplete EDTA-free Protease Inhibitor Cocktail /50ml (Roche)), and incubated for 30 min at 4˚C with gentle rotation. The total lysate was centrifuged at 12,000 g at 4˚C for 20 min twice to remove cell debris. Meanwhile, 30 μl Dynabeads Protein A (Ther-moFisher) was incubated with 8 μg of anti-HA antibody (Sigma, H6908) for 30 min at room temperature with gentle rotation, and the beads were then washed with lysis buffer 5 times to remove the extra antibody. The lysate was incubated with the anti-HA antibody-protein A beads for 2 hours at 4˚C with gentle rotation, and then beads were captured magnetically and washed with lysis buffer 5 times. Washed beads were divided for small RNA and protein analysis.
For small RNA analysis, washed beads were boiled in H 2 O for 5 min under constant shaking and removed magnetically. The supernatant containing immunoprecipitated RNA was used in small RNA library construction (as described above). For protein gel blot analysis, washed beads were boiled in 2xSDS sample buffer (50 mM Tris-HCL at pH 6.8, 10% glycerol, 2% SDS, 0.1% bromophenol blue, and 1% 2-mercaptoethanol) for 10 min followed by vigorous shaking. The beads were then removed magnetically, and the supernatant containing the immunoprecipitated protein was resolved in a 10% SDS-PAGE gel and detected by an anti-HA antibody (Roche, 12158167001).

RNA-seq and data analysis
Total RNA was isolated from 12-day-old Arabidopsis seedlings using TRI Reagent (NRC), and DNA was removed by DNase I (Roche) treatment. PolyA RNA was then enriched from 1 μg of DNase I-treated RNA using Oligo d(T)25 Magnetic Beads (NEB S1419S), followed by RNAseq libraries construction using the NEBNext Ultra Directional RNA Library Prep Kit for Illumina (NEB E7420). RNA libraries were sequenced on an Illumina NovaSeq 6000 platform (PE150 bp), and the sequencing data were analyzed using the pRNASeqTools v.0.8 pipeline.

Small RNA phasing analysis
Small RNA phasing analysis was performed as previously described [10,20,58]. Small RNA reads from both the sense and antisense strands were included in the analysis. The formula used for phasing score calculation was described before [10].

Nuclear-cytoplasmic fractionation
Nuclear-cytoplasmic fractionation was performed as described [37]. 1 g of 12-day-old Arabidopsis seedlings was crosslinked in 0.5% formaldehyde/1× PBS buffer through vacuum infiltration for 15 min at room temperature, and crosslinking was stopped by vacuum infiltration in 100 mM glycine/1× PBS buffer for 5 min at room temperature. The plant material was washed with 1× PBS buffer and blotted dry, and then ground to a fine powder in liquid nitrogen. The powder was resuspended in 2 ml lysis buffer (20 mM Tris-HCl, pH7.5, 20 mM KCl, 2.5 mM MgCl 2 , 2 mM EDTA, 25% glycerol, 250 mM sucrose, 5 mM DTT and cOmplete Protease Inhibitor Cocktail (Roche)) and filtered through a 40 μm cell strainer (Falcon). The flowthrough was centrifuged at 1,500g for 10 min at 4˚C. The supernatant representing the cytoplasmic fraction was further centrifuged at 10,000g for 10 min at 4˚C to remove any residual debris. The pellet from the 1,500g spin representing the nuclei was dissolved with 10 ml nuclei resuspension buffer 1 (NRB1) (20 mM Tris-HCl, pH7.4, 2.5 mM MgCl 2 , and 0.2% Triton X-100) and centrifuged at 1,500g for 10 min to collect nuclei; this step was repeated 8 times to thoroughly wash the nuclei. After the final wash, the pellet was resuspended with 500 μl NRB2 (20 mM Tris-HCl, pH7.5, 250 mM sucrose, 10 mM MgCl 2 , 0.5% Triton X-100, 5 mM 2-mercaptoethanol, and cOmplete Protease Inhibitor Cocktail (Roche)), and carefully loaded onto 500 μl NRB3 (20 mM Tris-HCl, pH7.5, 1.7 M sucrose, 10 mM MgCl 2 , 0.5% Triton X-100, 5 mM 2-mercaptoethanol, and cOmplete Protease Inhibitor Cocktail (Roche)) without disturbing the bottom layer, then the sample was centrifuged at 16,000g for 45 min at 4˚C. The nuclear pellet was resuspended and boiled in 1XSDS loading buffer for 10 min for protein gel blot analysis.

Correlation analysis
Pearson's correlation coefficients (R) and p-values were calculated in R (v4.2.2) with the function cor.test(x,y). Blots were probed using AGO1 or HA antibodies to detect AGO1. Histone H3 is a nuclear marker and used to quantify AGO1 in the total lysate (T) and the nuclear fraction (N). cFBPase is a cytoplasmic marker and used to quantify AGO1 in the T and the cytoplasmic fraction (C). The numbers represent relative protein levels. The arrows indicate the endogenous protein (AGO1) and the protein from the transgene (HA-AGO1Δ91-189 or GFP-AGO1mNES), with the two being distinguished by size. The phasing of small RNAs over the TAS transcripts in ago1-36 and ago1-36 expressing wild-type AGO1 or AGO1 NTE mutants. (B) RNA gel blot analysis of ta-siRNA abundance in 12-day-old seedlings of ago1-3, ago1-36, and ago1-36 expressing AGO1 full-length or various NTE-truncated forms. The numbers represent ta-siRNA abundance in different genotypes relative to AGO1 ago1-36. The U6 blots serve as a loading control for the miRNA blots above. (C) The phasing (top) and the abundance (bottom) of small RNAs over AGO1, PPR, and NBS-LRR gene transcripts in ago1-36 and ago1-36 expressing wild-type AGO1 or AGO1 NTE mutants. (D) Bar plots showing the log 2 (fold change) of RNAs from TAS loci and several protein-coding genes between ago1-36, ago1-36 expressing various AGO1 mutants, and AGO1 ago1-36. Blue bars denote RNA with significantly different expression (fold change > 1.5 and adjust P value < 0.01). (TIF) S1